Slurries formed by crushing solid fuels such as coal and then adding water and additives are known as CWM or CWF (Coal Water Mixture/Coal Water Fuel), and are consequently attracting considerable attention as new fuels.
From the viewpoint of handling, a slurry fuel requires a viscosity of no more than 1,500 mPa·s (rotary viscometer, 25° C., shear rate value of 100 [1/sec], these settings also apply below). Furthermore, with the demand in recent years for higher heating values and higher combustion efficiency, heating values of at least 16.5 MJ/kg (4,000 kcal/kg) are required.
The increase in carbon dioxide gas emissions as a result of the huge consumption of fossil fuels is a significant cause of global warming, and is leading to increased pressure for reductions in carbon dioxide gas emissions. Biomass, including materials such as timber, is a non-fossil based renewable energy considered to produce zero carbon dioxide emissions, and because the ash content and the sulfur content are extremely low, the investment costs for combustion facilities can be reduced.
Timber thinnings, wood scraps from wood processing, prunings from roadside trees, bagasse, rice straw, and used paper are largely unused, and are either dumped or disposed of for a fee, and if these types of materials could be used as fuels, then it would enable effective use of unused organic resources. These unused organic resources are solids of a variety of different forms, and if these solids could be liquefied or converted to a slurry in a similar manner to coal, then a significant expansion in the range of possible uses could be expected.
With these circumstances in mind, at the 15th International Conference on Coal and Slurry Technology in 1990, the Energy and Environmental Research Center at the University of North Dakota reported the generation of a slurry fuel by hot water treatment of timber.
However, the solid fraction concentration of the slurry reported by the University of North Dakota was no more than a maximum of approximately 48 mass %, and slurries of higher concentrations could not be produced. At a solid fraction concentration of approximately 48 mass %, the heating value of the slurry is only approximately 3,400 kcal/kg. If an attempt is made to increase the solid fraction concentration in order to increase the heating value, then the slurry solidifies and cannot be handled as a slurry.
Gasification of these unused organic resources of biomass origin by partial oxidation reactions, and subsequent use as gas fuels or synthetic gases for chemical reactions is also being investigated.
In the case of a direct gasification of a biomass, if the reaction temperature is less than 800° C., then the quantities of tar, soot and char produced increase, and operation of the gasification furnace becomes difficult. As a result, the partial oxidation reaction temperature must be maintained at a high temperature of at least 800° C. In order to maintain the partial oxidation reaction temperature at a high temperature of at least 800° C., the quantity of oxygen supplied must be increased, and in such cases the usage efficiency of the coolant gas decreases. A further problem arose in that the concentration of H2 and CO, which represent the active ingredients within the targeted product gas, also decreases.
Furthermore, in a method in which a raw material biomass is crushed to form chips, because the production of chips smaller than a certain size is impossible, performing the gasification reaction within a pressurized system was problematic. In addition, because the biomass cannot be reduced to small enough particles, the rate of the partial oxidation reaction by oxygen is slow.